This invention relates generally to optical devices and particularly to a device for providing a substantially flat light field to a light imaging medium.
Many types of measuring and testing equipment require the recording of a light image on a light imaging medium. Frequently the recording medium is a CCD (charge coupled device), or a photo-diode array. In these types of solid state recording devices the individual sensing cells, or pixels, of the device are charged to various levels in proportion to the intensity of the impinging light. Typically, when making inspections, or tests, using a light imaging medium, a light image of the object being inspected is provided to an optical system including a focusing mechanism and a variable aperture. The optical system typically is a high quality commercially available camera. When a solid state recording device is used the film normally used in the camera is replaced by the solid state device. The pixels of the solid state device are charged to different levels proportional to the intensity of the impinging light. Accordingly, the pixel charge levels are an accurate electrical analog representation of the light image received from the object.
An inspection system utilizing a solid state imaging device is described in U.S. Pat. No. 4,454,545. The system described in this patent utilizes a camera having a CCD as the image recording medium. The charge levels on the various pixels are digitized and transferred to a storage mechanism where the digitized data are available for subsequent tests or measurements.
Many systems utilizing the projection of light images onto recording mediums for inspection or measuring are satisfactory for the purposes intended. However, because the operation of systems using solid state devices is dependent upon the response of the individual pixels, the devices must be calibrated prior to any actual use of the system. Typically, the solid state device is calibrated by exposing the device to a known intensity of light and detecting the individual pixel charge levels. Accordingly, it is preferable to uniformly illuminate all pixels of the device. One method of ensuring such uniform illumination entails the use of a commercially available flat field light box as the light source. Such light field boxes provide a light field which is significantly flatter than that of a standard light source. However, these boxes are not ideal because the light field cannot be adjusted or tailored to the system. Another difficulty arises because the light provided by the flat field light box is imaged onto the recording medium through an optical lensing system. As is known to those skilled in the art, the intensity of light passing through an optical system is significantly higher at the center of the focusing lens than at the periphery of the lens. For this reason the recording medium is not uniformly illuminated even when a flat field light box is used as the light source. Accordingly, the accurate calibration of solid state devices is extremely difficult and the useful range of such devices is restricted by the lack of a device for providing a substantially flat light field.
For these reasons there is a need for a device which provides a substantially flat light field to a recording medium and which permits adjustment and tailoring to the system employing the field to optimize the system. The present invention fulfills this long-felt need.